1. Field of the Invention
The present invention is directed towards a power source and, more particularly, to an improved resonant current driven power source of the type disclosed in copending U.S. patent application Ser. No. 417,465 of George C. Gallios, filed Sept. 13, 1982, U.S. Pat. No. 4,475,149 for a "Resonant Current-Driven Power Source", and assigned to the assignee of the instant invention. In the presently preferred embodiment, the power source is constructed as a DC to DC converter regulator having a relatively low input voltage and delivering relatively high output power.
2. Description of the Prior Art
As explained in greater detail in the aforesaid application Ser. No. 417,465, converters of the prior art fall into two primary categories: voltage-driven converters and current-driven converters. A typical voltage-driven converter comprises four switching transistors which are connected between a source voltage and the primary winding of a transformer. A control circuit applies inverse square waves to the transistors so they operate in a saturated square wave power "chopper" mode to induce an AC voltage across the primary winding of the transformer. This AC voltage has a peak-to-peak value of approximately twice the voltage of the DC source. The AC voltage across the primary winding of the transformer induces a stepped up or stepped down voltage (depending upon the turns ratio of the transformer) across the secondary winding of the transformer, which induce voltage is applied across a full wave bridge rectifier so as to charge a capacitor across the bridge output to the desired output voltage level.
Since there is no resistance in the charging path to the capacitor, the latter acts as a peak detector and stores an output voltage which is determined only by the voltage across the primary winding of the transformer and the turns ratio of the transformer. As such, the output voltage of this type of voltage converter is related to the input supply voltage by a constant ratio. This makes it impossible to electronically change the input to output voltage ratio. It thus prevents the voltage-driven converter circuit from varying the output voltage (for a given source voltage), and from compensating for variations in line voltage, or variations due to load regulation. As such, the voltage-driven converter cannot operate as a regulator.
In an effort to overcome some of the shortcomings of the voltage-driven converter, the prior art has developed various current driven inverters. A typical current driven inverter includes a charging capacitor that is placed across the output load rather than being connected directly across the output of the bridge. An inductor is placed in series with the capacitor and forms an integrating circuit in the output path of the diode bridge. As such, the magnitude of the output voltage appearing across the load can be modified by modifying the duration of the circuit pulses produced by diode bridge. The duration of the pulses can, in turn, be controlled by controlling the duty cycle of the driving pulses applied to the transistors. In this manner, the current-driven converter can operate as a regulator.
The primary drawbacks of the prior art current driven DC/DC converter are as follows. Initially, the integrating inductor must be large (and therefore expensive) since it must accommodate all of the DC current and store enough energy at all operating currents to maintain continuous output current flow. Furthermore, its resistance must be adequately low to avoid efficiency degrading losses and thermal problems.
The resonant current driven DC/DC converter regulator of the aforesaid copending application Ser. No. 417,465 comprises an inductor and a capacitor electrically coupled to one another; an input inverter circuit for converting an input DC voltage into an AC voltage having substantially no DC component, with the input inverter circuit applying the AC voltage across the inductor and capacitor in a manner which causes the inductor and capacitor to resonate with one another whereby an AC voltage appears across the capacitor; and an output circuit for converting the AC voltage appearing across the capacitor into a DC output voltage.
The most significant design change incorporated in that resonant current driven DC/DC converter regulator appears to be the introduction of the induction element into the AC arm of a four element transistor bridge defining the input inverter circuit. This eliminates DC flux in the inductor with the exception of unbalanced currents and allows a substantial reduction in the size and rating of the inductor. By providing the inductive element in the AC arm of the inverter circuit, the effect of reactive loads are minimized and the inverter can be readily controlled from the maximum output level down to no load by proper pulse width modulation of the transistors defining the inverter circuit.
As will hereinafter be seen, the resonant current driven converter circuit of copending application Serial No. 417,465 includes a four element transistor bridge, the top and bottom nodes of which receive the source voltage and the left and right nodes of which apply an AC current across the LC resonant circuit. The two lower transistors receive respective square waves which are inverted with respect to one another and which have a frequency fs=1/Ts which defines the frequency of the AC voltage appearing across the LC circuit.
The two upper transistors receive respective pulse width modulated control signals which are inverted with respect to one another and which turn each of the two transistors on during only a portion of a respective half cycle of the switching period Ts. A diode bridge is connected across the capacitor of the LC resonant circuit and transfers energy to an output capacitor located between two nodes of the diode bridge. Whenever the voltage across the resonant capacitor rises to a level equal to that of the power supply output voltage Eo, energy is transferred via the resonant circuit to the output capacitor. By selecting the resonant frequency of the LC circuit to be substantially higher (e.g., two and one-half times) than the switching frequency fs of the two lower transistors, it is possible to control the amount of current supplied to the output capacitor by varying the duty cycle of the pulses applied to the two upper transistors of the transistor bridge. Particularly, the duty cycle of the control pulses applied to the upper transistors varies the magnitude and duration of charging current applied to the output capacitor located in the diode bridge and thereby controls the level of the output voltage Eo. This "product" control of pulse duration and amplitude provides excellent regulation at any current within the device's maximum current capability. The current through the LC circuit has a wave shape approximating a trapezoid and an output characteristic approximating a constant current source. Since the output circuit is effectively being driven by a constant current course, it is intrinsically capable of being short-circuited on the output.
As will hereinafter be seen, in accordance with the instant invention the resonant current driven power source of the aforesaid copending application Ser. No. 417,465 will operate more efficiently, and with lower peak, average and rms currents at lower input supply voltages, particularly where the power levels are high.
In most, if not all regulators, the electrical current from the input power supply is determined by the input power and by the input voltage. Where the supply voltages are low, and the input currents are high, it becomes difficult and/or expensive to efficiently convert power. For example, to convert 10 amperes from a 200 volt supply (2,000 watts) to 1,800 watts output (90% efficiency) is readily achievable. To convert 100 amperes from a 20 volt supply, again 2,000 watts, to 1,800 watts output is more difficult because of the losses, particularly in the semi-conductors, at the higher currents.